Red giant stars—such as those speckling the Messier 79 globular cluster in this Hubble Space Telescope image—are providing new constraints on the expansion rate of the universe. Credit: NASA, ESA, STScI, F. Ferraro University of Bologna and S. Djorgovski California Institute of Technology

Researchers hoped new data would resolve the most contentious question in cosmology. They were wrong
By Leila Sloman on July 29, 2019

How fast is the universe expanding?

One might assume scientists long ago settled this basic question, first explored nearly a century ago by Edwin Hubble. But right now the answer depends on who you ask. Cosmologists using the Planck satellite to study the cosmic microwave background—light from the “early” universe, only about 380,000 years after the big bang—have arrived at a high-precision value of the expansion rate, known as the Hubble constant (H0). Astronomers observing stars and galaxies closer to home—in the “late” universe—have also measured H0 with extreme precision. The two numbers, however, disagree. According to Planck, H0 should be about 67—shorthand for the universe expanding some 67 kilometers per second faster every 3.26 million light-years. The most influential measurements of the late universe, coming from a project called Supernova H0 for the Equation of State (SH0ES), peg the Hubble constant at about 74.

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Peter van Nieuwenhuizen, Sergio Ferrara and Dan Freedman (left to right, in a picture from 2016) received a Breakthrough Prize for creating supergravity theory.Credit: CERN

Three physicists honoured for theory that has been hugely influential — but might not be a good description of reality.

Whether the theory of supergravity, an attempt to unify all the forces of nature, is a true description of the world still hangs in the balance more than 40 years after it was proposed. Nonetheless it has now nabbed its founders one of the most lucrative awards in science: a shared US$3-million Special Breakthrough Prize in fundamental physics.

Supergravity1 was devised in 1976 by particle physicists Sergio Ferrara of CERN, Europe’s particle-physics laboratory near Geneva, Switzerland; Daniel Freedman of the Massachusetts Institute of Technology in Cambridge; and Peter van Nieuwenhuizen of Stony Brook University in New York. The selection committee that awarded the prize chose to honour the theory, in part, for its impact on the understanding of ordinary gravity. Supergravity also underpins one of physicists’ favourite candidate ‘theories of everything’, string theory. The latter asserts that elementary particles are made of tiny threads of energy, but it remains unproven.

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                      Mike Zeng for Quanta Magazine

Natalie Wolchover - June 6, 2019

A recent challenge to Stephen Hawking’s biggest idea — about how the universe might have come from nothing — has cosmologists choosing sides.

In 1981, many of the world’s leading cosmologists gathered at the Pontifical Academy of Sciences, a vestige of the coupled lineages of science and theology located in an elegant villa in the gardens of the Vatican. Stephen Hawking chose the august setting to present what he would later regard as his most important idea: a proposal about how the universe could have arisen from nothing.

Before Hawking’s talk, all cosmological origin stories, scientific or theological, had invited the rejoinder, “What happened before that?” The Big Bang theory, for instance — pioneered 50 years before Hawking’s lecture by the Belgian physicist and Catholic priest Georges Lemaître, who later served as president of the Vatican’s academy of sciences — rewinds the expansion of the universe back to a hot, dense bundle of energy. But where did the initial energy come from?

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Credit: Event Horizon Telescope Collaboration

Nature Physics 15, 415 (2019) -- Published: 02 May 2019

General relativity was first experimentally verified in 1919. On the centennial of this occasion, we celebrate the scientific progress fuelled by subsequent efforts at verifying its predictions, from time dilation to the observation of the shadow of a black hole.

When we think back to the beginnings of Einstein’s general theory of relativity, we consider the measurements obtained during the solar eclipse in 1919 as rock-solid proof. However, things weren’t as clear cut back then. In 1921, the editorial introducing the special issue on general relativity in Nature (https://go.nature.com/2uZCo4E) betrayed a certain level of caution: “In two cases predicted phenomena for which no satisfactory alternative explanation is forthcoming have been confirmed by observation, and the third is still a subject of inquiry.”

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Total solar eclipse, 29 May 1919. Glass positive photograph of the corona, taken at Sobral in Brazil, with a telescope of 4in in aperture and 19ft focal length. Photograph: Science & Society Picture Library/SSPL via Getty Images

Robin McKie
Sun 12 May 2019 

Arthur Eddington’s photographs of the 1919 solar eclipse proved Einstein right and ushered in a century where gravity was king

A hundred years ago this month, the British astronomer Arthur Eddington arrived at the remote west African island of Príncipe. He was there to witness and record one of the most spectacular events to occur in our heavens: a total solar eclipse that would pass over the little equatorial island on 29 May 1919.

Observing such events is a straightforward business today, but a century ago the world was still recovering from the first world war. Scientific resources were meagre, photographic technology was relatively primitive, and the hot steamy weather would have made it difficult to focus instruments. For good measure, there was always a threat that clouds would blot out the eclipse.

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The expanding Universe, full of galaxies and the complex structure we observe today, arose from a smaller, hotter, denser, more uniform state. It took thousands of scientists working for hundreds of years for us to arrive at this picture, and yet the lack of a consensus on what the expansion rate actually is tells us that either something is dreadfully wrong, we have an unidentified error somewhere, or there’s a new scientific revolution just on the horizon. (C. FAUCHER-GIGUÈRE, A. LIDZ, AND L. HERNQUIST, SCIENCE 319, 5859 (47))

How fast is the Universe expanding? The results might be pointing to something incredible.

Ethan Siegel  May 10, 2019

If you want to know how something in the Universe works, all you need to do is figure out how some measurable quantity will give you the necessary information, go out and measure it, and draw your conclusions. Sure, there will be biases and errors, along with other confounding factors, and they might lead you astray if you’re not careful. The antidote for that? Make as many independent measurements as you can, using as many different techniques as you can, to determine those natural properties as robustly as possible.
If you’re doing everything right, every one of your methods will converge on the same answer, and there will be no ambiguity. If one measurement or technique is off, the others will point you in the right direction. But when we try to apply this technique to the expanding Universe, a puzzle arises: we get one of two answers, and they’re not compatible with each other. It’s cosmology’s biggest conundrum, and it might be just the clue we need to unlock the biggest mysteries about our existence.

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BLAST FROM COLLAPSE A collapsar occurs when a massive, spinning star collapses into a black hole, powering a blast of light known as a long gamma ray burst (illustrated) and exploding the star’s outer layers.

Spinning stellar objects collapsing into black holes could help explain heavy elements’ origins
BY EMILY CONOVER  MAY 8, 2019

The gold in your favorite jewelry could be the messy leftovers from a newborn black hole’s first meal.

Heavy elements such as gold, platinum and uranium might be formed in collapsars — rapidly spinning, massive stars that collapse into black holes as their outer layers explode in a rare type of supernova. A disk of material, swirling around the new black hole as it feeds, can create the conditions necessary for the astronomical alchemy, scientists report online May 8 in Nature.

“Black holes in these extreme environments are fussy eaters,” says astrophysicist Brian Metzger of Columbia University, a coauthor of the study. They can gulp down only so much matter at a time, and what they don’t swallow blows off in a wind that is rich in neutrons — just the right conditions for the creation of heavy elements, computer simulations reveal.

Astronomers have long puzzled over the origins of the heaviest elements in the universe. Lighter elements like carbon, oxygen and iron form inside stars, before being spewed out in stellar explosions called supernovas. But to create elements further down the periodic table, an extreme environment densely packed with neutrons is required. That’s where a chain of reactions known as the r-process can occur, in which atomic nuclei rapidly absorb neutrons and undergo radioactive decay to create new elements.

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NASA’S GODDARD SPACE FLIGHT CENTER/CI LAB

Astronomers still hope to catch a star going supernova and a bumpy neutron star, among others
BY LISA GROSSMAN 1:14PM, MAY 6, 2019

BANG, CRASH Physicists using the LIGO and Virgo observatories are catching all sorts of cosmic collisions, including of pairs of neutron stars (illustrated). But scientists hope to bag even more exotic quarry.

Seekers of gravitational waves are on a cosmic scavenger hunt.

Since the Advanced Laser Interferometer Gravitational-wave Observatory turned on in 2015, physicists have caught these ripples in spacetime from several exotic gravitational beasts — and scientists want more.

This week, LIGO and its partner observatory Virgo announced five new possible gravitational wave detections in a single month, making what was once a decades-long goal almost commonplace (SN Online: 5/2/19).

“We’re just beginning to see the field of gravitational wave astronomy open,” LIGO spokesperson Patrick Brady from the University of Wisconsin–Milwaukee said May 2 in a news conference. “Opening up a new window on the universe like this will hopefully bring us a whole new perspective on what’s out there.”

The speed and pitch of gravitational wave signals allow astronomers to make out what’s stirring up the waves. Here are the sources of gravitational waves that scientists that already have in their nets, and what they’re still hoping to find.

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Scientific simulation of a black hole consuming a neutron star.Credit: A. Tonita, L. Rezzolla, F. Pannarale

NATURE 26 APRIL 2019

Davide Castelvecchi

Gravitational waves may have just delivered the first sighting of a black hole devouring a neutron star. If confirmed, it would be the first evidence of the existence of such binary systems. The news comes just a day after astronomers had detected gravitational waves from a merger of two neutron stars for only the second time.

At 15:22:17 UTC on 26 April, the twin detectors of the Laser Interferometer Gravitational-wave Observatory (LIGO) in the United States and the Virgo observatory in Italy reported a burst of waves of an unusual type. Astronomers are still analysing the data and doing computer simulations to interpret them.

But they are already considering the tantalizing prospect that they have made a long-hoped-for detection that could produce a wealth of cosmic information, from precise tests of the general theory of relativity to measuring the Universe’s rate of expansion. Astronomers around the world are also racing to observe the phenomenon using different types of telescope.

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EHT images of M87 on four different observing nights. In each panel, the white circle shows the resolution of the EHT. All four images are dominated by a bright ring with enhanced emission in the south. 

The Astrophysical Journal Letters

Shep Doeleman (EHT Director) on behalf of the EHT Collaboration -- April 2019

This Focus Issue shows ultra-high angular resolution images of radio emission from the supermassive black hole believed to lie at the heart of galaxy M87 (Figure 1). A defining feature of the images is an irregular but clear bright ring, whose size and shape agree closely with the expected lensed photon orbit of a 6.5 billion solar mass black hole. Soon after Einstein introduced general relativity, theorists derived the full analytic form of the photon orbit, and first simulated its lensed appearance in the 1970s. By the 2000s, it was possible to sketch the "shadow" formed in the image when synchrotron emission from an optically thin accretion flow is lensed in the black hole's gravity. During this time, observational evidence began to build for the existence of black holes at the centers of active galaxies, and in our own Milky Way. In particular, a steady progression in radio astronomy enabled very long baseline interferometry (VLBI) observations at ever-shorter wavelengths, targeting supermassive black holes with the largest apparent event horizons: M87, and Sgr A* in the Galactic Center. The compact sizes of these two sources were confirmed by studies at 1.3mm, first exploiting baselines that ran from Hawai'i to the mainland US, then with increased resolution on baselines to Spain and Chile.

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Mollweide projection of bayestar.fits bayestar.png. Submitted by LIGO/Virgo EM Follow-Up on Apr 8, 2019 18:52:21 UTC

A week into the 3rd @LIGO @ego_virgo Observing Run and our first candidate has been posted to GraceDB - the #GravitationalWave candidate event database: say hello to S190408an! More info, including a map of its likely sky location, at https://gracedb.ligo.org/superevents/S190408an/view/ …

Stay tuned this week for more about our #GravitationalWave candidate events: how often to expect them, how to access the alert info and what it means. And remember, we're expecting *lots* of #O3 events and the alerts will be public! https://emfollow.docs.ligo.org/userguide/

GraceDB — Gravitational Wave Candidate Event Database

Credit: (Griffith University)

DAVID NIELD 20 MAR 2019

In quantum physics, particles can 'tunnel' through seemingly impenetrable barriers, even when they apparently don't have the energy to do so. Now, researchers have gleaned behind the curtain to better understand how this trick is done.

This problem has puzzled scientists for decades – in particular, the time it takes for particles to do their quantum tunnelling, and get from one side of a barrier to another.

In the case of the atomic hydrogen particles used in these experiments, the researchers found that it happens instantaneously.

That instant transmission had been investigated before, but now scientists have observed it in the lab by using a special piece of equipment called an attoclock: a device that uses the properties of light to measure particle progress.

"We use the simplest atom, atomic hydrogen, and we've found that there's no delay in what we can measure," says one of the team, Robert Sang from Griffith University in Australia.

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Credit: (agsandrew/iStock)

MIKE MCRAE 31 MAR 2019

Today our middle-aged Universe looks eerily smooth. Too smooth, in fact.

While a rapid growth spurt in space-time would explain what we see, science needs more than nice ideas. It needs evidence that whittles away contending arguments. We might finally know where to look for some.

A team of physicists from the Centre for Astrophysics | Harvard & Smithsonian (CfA) and Harvard University went back to the drawing board on the early Universe's evolution to give us a way to help those inflation models stand out from the crowd.

"The current situation for inflation is that it's such a flexible idea, it cannot be falsified experimentally," says theoretical physicist Avi Loeb from the CfA.

"No matter what value people measure for some observable attribute, there are always some models of inflation that can explain it."

We've been convinced for some time that our Universe is expanding – its fabric slowly stretching out under the influence of some kind of strange 'dark' energy.

If we press rewind on the Universe until it was barely 10^-43 seconds old, we arrive at the limit of what our knowledge of physics can handle. Before that moment? Geometry is so nuts, we just don't know where to start.

Running the calculations backward, we also find the Universe would have had a radius of 10^-10 metres at this crucial moment. That sounds tiny, sure, but it's not tiny enough.

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Credit: Kavli IPMU

MICHELLE STARR 4 APR 2019

We still don't know what dark matter is, but we can strike a line through one option. It is not, as per a theory proposed by the brilliant Stephen Hawking, a bunch of teeny-tiny microscopic black holes.

In the most rigorous test of the theory to date, an international team led by researchers from the Kavli Institute for the Physics and Mathematics of the Universe (IPMU) in Japan has searched for the telltale sign of such minuscule black holes, and the result was pretty damning.

The scientists were hunting for a particular flicker of stars in a nearby galaxy - the way the light would appear to us if a black hole less than a tenth of a millimetre were passing in front of it.

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The most-visualized black hole of all, as illustrated in the movie Interstellar, shows a predicted event horizon fairly accurately for a very specific class of rotating black holes. Deep within the gravitational well, time passes at a different rate for observers than it does for us far outside of it. The Event Horizon Telescope is expected to reveal the emissions surrounding a black hole's event horizon, directly, for the first time. INTERSTELLAR / R. HURT / CALTECH

Ethan Siegel
Apr 2, 2019

In science, there's no moment more exciting than when you get to confront a longstanding theoretical prediction with the first observational or experimental results. Earlier this decade, the Large Hadron Collider revealed the existence of the Higgs boson, the last undiscovered fundamental particle in the Standard Model. A few years ago, the LIGO collaboration directly detected gravitational waves, confirming a longstanding prediction of Einstein's General Relativity.

And in just a few days, on April 10, 2019, the Event Horizon Telescope will make a much-anticipated announcement where they're expected to release the first-ever image of a black hole's event horizon. At the start of the 2010s, such an observation would have been technologically impossible. Yet not only are we about to see what a black hole actually looks like, but we're about to test some fundamental properties of space, time, and gravity as well.

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